1. Field of the Invention
This invention relates to the inorganic surface modification of an aluminum oxide supported rhodium catalyst that forms the basis for an improved automotive catalytic converter useful in environmental protection.
2. Brief Description of the Prior Art
Automotive catalysts for controlling automotive exhaust emissions are comprised of the noble metals platinum (Pt), palladium (Pd), and rhodium (Rh). Of these metals, rhodium is the most effective for reducing the oxides of nitrogen to nitrogen gas. Generally in the automotive catalyst field, rhodium is highly dispersed on aluminum oxide (Al.sub.2 O.sub.3) supports and is active as a metal particle designated Rh.sub.x .sup.(O) where the subscript "x" refers to the number of Rh atoms in a Rh crystallite or assembly and the superscript "(O)" refers to the oxidation state number. It is known that active Rh.sub.x .sup.(O) may be converted to an oxidized species Rh.sup.(I) by an oxidation process in which surface hydroxyl groups present on the oxide support are consumed in the presence of carbon monoxide (CO) gas. Both the carbon monoxide gas and the surface hydroxyl groups simultaneously participate in the destruction of the Rh.sub.x .sup.(O). This process can be reversed using hydrogen gas (H.sub.2) which reduces the Rh.sup.(I) surface species, present as Rh.sup.(I) (CO).sub.2 species, to Rh.sub.x .sup.(O) while regenerating the isolated hydroxyl groups. J. Am. Chem. Soc., Vol. 110, p. 2074 (1988).
U.S. Pat. No. 3,271,325 discloses a composition for an alumina containing material for use in catalysts having a hydrate of alumina, aluminum, and a compound of a metal selected from aluminum and metals of Groups I, II, III and VIII, of the Periodic Table. This patent discloses that the metal may be, among other things, platinum group metal hydroxides and carbonates. It discloses that a steam reforming catalyst composition may be prepared in a variety of ways and that the platinum group metal constituents of the catalyst, such as for example rhodium, may be incorporated at various stages in its preparation. It also states that it is useful to have present in a catalyst composition an alkali metal compound such as potassium carbonate to reduce the formation of carbon in a steam reforming process.
U.S. Pat. No. 3,629,153 discloses a process for preparing an alkalized alumina solid adsorbent by first reacting an alkali metal aluminate with either ammonium carbonate or carbon dioxide to form dawsonite and then heating the dawsonite to convert it to alkalized alumina comprising substantially Al.sub.2 O.sub.3 and an alkali metal.
U.S. Pat. No. 3,752,775 discloses an oxidation catalyst for catalyzing oxidation reactions in hydrocarbon-oxygen containing mixtures having an alkali metal polyaluminate wherein the alkali metal may be potassium. It discloses a process for producing an oxidation catalyst having an alkali polyaluminate. It also states that this catalyst is especially suitable for reactions which are liable to cause problems by formation of free carbon, such as in the conversion of a high calorific oil gas containing relatively large amounts of unsaturated hydrocarbons and in the cleaning of exhaust gas from automobiles containing a large amount of un-ignited fuel.
U.S. Pat. No. 3,948,809 discloses a process for producing an absorbent by leaching a material containing aluminum oxide with an alkaline solution, separating the resultant leach liquor from the undissolved material, adding an alkali metal carbonate to the separated leach liquor, drying the liquor to produce the absorbent in solid form and calcining the absorbent.
U.S. Pat. No. 4,083,919 discloses a hot-pressed beta-alumina composition prepared by mixing and reacting a solution of aluminum alcoholate and an aqueous solution of an alkali metal salt to form a gelatinous precipitate, drying the gelatinous precipitate, grinding the dried gelatinous precipitate into a fine powder and hot-pressing the fine powder. This patent states that the alkali metal salt may be potassium carbonate.
U.S. Pat. No. 4,207,169 discloses a process for steam dealkylation of alkylaromatic hydrocarbons employing a catalyst containing an alumina carrier, rhodium, titanium oxide, and an alkali metal that may be in the form of potassium carbonate. This patent states that the addition of titanium oxide to the alumina carrier has the effect of reducing the deactivation rate of the catalyst.
U.S. Pat. No. 4,755,499 discloses a sorbent for the removal of gaseous nitrogen oxides, sulfur oxides, and hydrogen sulfide from waste gas streams. The sorbent is a gamma alumina substrate impregnated with an alkali metal component wherein the alkali metal component may be potassium.
In spite of these prior art disclosures, there remains a very real and substantial need for a process for the chemical modification of an aluminum oxide supported rhodium catalyst to prevent conversion of Rh.sub.x .sup.(O) to Rh.sup.(I) surface species. Such a process may be employed to create an automotive catalyst converter which is beneficial for environmental protection, as well as stable Rh.sub.x .sup.(O) catalysts used for other chemical processes such as for example benzene reduction to cyclohexane and organic nitrile reduction to organic amines. More specifically, there is a need for such a chemical modification of alumina using an inorganic cation, Y, wherein Y may be either an alkali metal or an alkaline earth metal. Here surface hydroxyl groups on the aluminum oxide support are converted to Al--OY groups. This prevents the conversion of active rhodium Rh.sub.x .sup.(O) species to inactive rhodium species, Rh.sup.(I), by removal of the hydroxyl groups which otherwise would participate in Rh.sub.x .sup.(O) .fwdarw.Rh.sup.(I) conversion.